In an Electric Power Steering (EPS) unit an electric motor drives a worm shaft and worm gear to provide assist torque to the turning of a steering shaft. This reduces the effort required to steer a vehicle. Currently worm gears used in these systems have been made using a solid steel puck. Each puck is then machined with a knurl on the perimeter. The puck then is the base or hub of the worm gear assembly.
The knurled surface is bead blasted to prep for a silane solution treatment that prepares the metal for bonding. A ring of plastic, made by a spin cast method, is placed on the metal. After the plastic is pressed on, the worm gear assembly is heated to cause the plastic to melt into the knurl surface of the hub and bond to the steel. This is followed by an annealing cycle to stress relieve the plastic. The hub assembly is pressed onto a shaft and teeth are hobbed (or cut) into the plastic ring to complete the gear assembly. As used in an electric power steering application or other application, the knurl to plastic bond transfers assist torque from the worm shaft, through the worm gear assembly, to the steering shaft. The process of making one gear hub assembly can be found in U.S. Pat. No. 6,638,390.
Machining of gear hubs to create the knurled surface with which to bond the plastic requires additional processing and handling.
Accordingly, it is desirable to provide a worm gear hub and worm gear hub assembly capable of transferring torque between a worm shaft and a steering shaft without the prior disadvantages.